a) Field of the invention
The present invention relates to a variable magnification viewfinder to be used with photographic cameras, video cameras and so on.
b) Description of the prior art
The inverted Galilean finder is known as a viewfinder comprising an imaging lens system and a finder optical system which are designed as systems separate from each other. However, the inverted Galilean finder has defects that it cannot form a clear image of a frame of a visual field and that it allows observation of the visual field itself to be hindered due to the ghost and flare produced by a half mirror used for forming the image of the frame of the visual field.
In contrast, the Keplerian finder allows observation of a clear image of object since it is designed so as to allow observation of a real image formed by an objective lens system through an eyepiece lens system.
Further, as Keplerian finders having variable magnification functions, there are known a finder which is equipped with an objective lens system designed as a zoom lens system composed of two lens units and another finder which is equipped with an objective lens system designed as a zoom lens system composed of three lens units. The former type of conventional finder is exemplified by those disclosed by Japanese Patents Kokai Publication No. Sho 61-156018, Kokai Publication No. Sho 64-65519 and Kokai Publication No. Hei 1-257817, thereas the latter type of conventional finder is exemplified by that disclosed by Japanese Patent Kokai Publication No. Hei 1-131510.
However, each of the above-mentioned conventional examples is undesirable since it uses an objective lens system having a long total length and enlarges a camera when the finder is assembled with the camera.
In addition, as a further example of the conventional finder comprising an objective lens system composed of two lens units, there is known the finder disclosed by Japanese Patent Kokai Publication No. Sho 61-156019. The objective lens system of this conventional example is composed of a positive lens unit and a negative lens unit, and has a short total length, but requires a high manufacturing cost since it uses glass materials having high refractive indices for correcting aberrations favorably.